414 NERVOUS SYSTEM. 



Humours, the Vitreous is, by far, the most abundant, and con- 

 stitutes a principal part of the eyeball; the Crystalline Humour 

 is in front of the vitreous; and the Aqueous is placed between 

 the crystalline and the cornea. 



Tunics, or Membranes of the Eyeball. 



The Sclerotic Coat (Tunica Sclerotica, Jllbugineo) ' forms 

 about five-sixths of the exterior investment of the eyeball, the 

 remaining sixth of which is obtained from the cornea. At its 

 posterior part it is joined by the optic nerve: this junction does 

 not occur precisely at its axis or centre, but at the inner side of 

 if. When the optic nerve is detached at this junction, a small 

 round hole is perceptible in the sclerotica, or, rather, it is more 

 frequently perceived as a thin cribriform lamella, through the 

 holes of which the pulpy part of the optic nerve passes, so as 

 to get within' the eye. This cribriform lamella, or the appear- 

 ance of it, is no doubt produced artificially by the nerve being 

 commonly cut through very near the eye; and, as Mr. Jacobs, 

 of Dublin, very properly suggests, should he considered as the 

 most anterior termination, or the point of the optic nerve in- 

 stead of as a portion of the sclerotica. The neurileme of the 

 optic nerve, is so arranged, that small round longitudinal canals 

 are left, which contain the nervous matter : from this cause it 

 happens that a thin section of the optic nerve in any part of its 

 course in the orbit, will, if held up to the light, manifest the 

 same cribriform arrangement with the part alluded to. This 

 part of the structure of the optic nerve will be readily under- 

 stood by the American student, in comparing it with the pith 

 of the Indian corn-stalk, which, being traversed longitudinally, 

 by many fibres, upon the drawing of them out, an equal number 

 of longitudinal canals is left in their places. 



At its fore part, the edge of the sclerotica is bevelled all 

 around for its junction with the cornea; and though nearly 

 circular, is not completely so, from its horizontal diameter being 

 somewhat greater than any other. There are several orifices 

 of inconsiderable size scattered over the sclerotica, some of 

 which are oblique and others direct: they transmit the blood 

 vessels and nerves. The Sclerotica is nearly a line in thick- 

 ness at its back part, from which it gradually becomes reduced 

 to half that thickness in front, where it is strengthened by the 



